A subdomain is the section of the web address that's before a domain name and you've most likely seen a lot of subdomains while surfing around the Web. As an illustration, many sites such as Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The advantage of employing a subdomain is that it can have an independent website and its own records, so you can even host it on a separate server. The practical use is that you could have a supplementary site, like an e-learning portal for college students aside from the primary school website. If you work with subdomains rather than subfolders, it's going to be much easier to perform maintenance or to upgrade a specific site, not mentioning that it's going to be more secure to have the sites separate from one another.